C Language Tutorial
This section contains a brief introduction to the C language. It is intended
as a tutorial on the language, and aims at getting a reader new to C started as
quickly as possible. It is certainly not intended as a substitute for
any of the numerous textbooks on C.
The best way to learn a new ``human'' language is to speak it right from the
outset, listening and repeating, leaving the intricacies of the grammar for
later. The same applies to computer languages--to learn C, we must start writing
C programs as quickly as possible.
An excellent textbook on C by two well-known and widely respected authors is:
The C Programming Language -- ANSI C Brian W. C. Kernighan & Dennis M. Ritchie Prentice Hall, 1988Dennis Ritchie designed and implemented the first C compiler on a PDP-11
(a prehistoric machine by today's standards, yet one which had enormous
influence on modern scientific computation). The C language was based on two
(now defunct) languages: BCPL, written by Martin Richards, and B, written by Ken
Thompson in 1970 for the first UNIX system on a PDP-7. The original ``official''
C language was the ``K & R'' C, the nickname coming from the names of the
two authors of the original ``The C Programming Language''. In 1988, the
American National Standards Institute (ANSI) adopted a ``new and improved''
version of C, known today as ``ANSI C''. This is the version described in the
current edition of ``The C Programming Language -- ANSI C''. The ANSI version
contains many revisions to the syntax and the internal workings of the language,
the major ones being improved calling syntax for procedures and standarization
of most (but, unfortunately, not quite all!) system libraries.
1. A First Program
Let's be polite and start bysaluting the world! Type the following program into your favorite editor:
Save the code in the file hello.c, then compile
#include < stdio.h> void main() { printf("\nHello World\n"); }
it by typing:
gcc hello.c
This creates an executable file a.out, which is then
executed simply by typing its name. The result is that the characters ``
Hello World'' are printed out, preceded by an empty line.
A C program contains functions and variables. The functions
specify the tasks to be performed by the program. The ``main'' function
establishes the overall logic of the code. It is normally kept short and calls
different functions to perform the necessary sub-tasks. All C codes must have a
``main'' function.
Our hello.c code calls printf, an output function from
the I/O (input/output) library (defined in the file stdio.h). The
original C language did not have any built-in I/O statements whatsoever. Nor did
it have much arithmetic functionality. The original language was really not
intended for ''scientific'' or ''technical'' computation.. These functions are
now performed by standard libraries, which are now part of ANSI C. The K & R
textbook lists the content of these and other standard libraries in an appendix.
The printf line prints the message ``Hello World'' on
``stdout'' (the output stream corresponding to the X-terminal window
in which you run the code); ``\n'' prints a ``new line'' character,
which brings the cursor onto the next line. By construction, printf
never inserts this character on its own: the following program would produce the
same result:
Try leaving out the ``\n'' lines and see what happens.
#include < stdio.h> void main() { printf("\n"); printf("Hello World"); printf("\n"); }
The first statement ``#include < stdio.h>'' includes a
specification of the C I/O library. All variables in C must be explicitly
defined before use: the ``.h'' files are by convention ``header
files'' which contain definitions of variables and functions necessary for the
functioning of a program, whether it be in a user-written section of code, or as
part of the standard C libaries. The directive ``#include'' tells the
C compiler to insert the contents of the specified file at that point in the
code. The ``< ...>'' notation instructs the compiler to look
for the file in certain ``standard'' system directories.
The void preceeding ``main'' indicates that main is of
``void'' type--that is, it has no type associated with it, meaning that
it cannot return a result on execution.
The ``;'' denotes the end of a statement. Blocks of statements are put in
braces {...}, as in the definition of functions. All C statements are defined in
free format, i.e., with no specified layout or column assignment. Whitespace
(tabs or spaces) is never significant, except inside quotes as part of a
character string. The following program would produce exactly the same result as
our earlier example:
#include < stdio.h>
void main(){printf("\nHello World\n");}
The reasons for arranging your programs in lines and indenting to show
structure should be obvious!
2. Let's Compute
The following program,sine.c, computes a table of the sine function for angles between 0
and 360 degrees.
The code starts with a series of comments indicating its the purpose, as
/************************/ /* Table of */ /* Sine Function */ /************************/ /* Michel Vallieres */ /* Written: Winter 1995 */ #include < stdio.h> #include < math.h> void main() { int angle_degree; double angle_radian, pi, value; /* Print a header */ printf ("\nCompute a table of the sine function\n\n"); /* obtain pi once for all */ /* or just use pi = M_PI, where M_PI is defined in math.h */ pi = 4.0*atan(1.0); printf ( " Value of PI = %f \n\n", pi ); printf ( " angle Sine \n" ); angle_degree=0; /* initial angle value */ /* scan over angle */ while ( angle_degree <= 360 ) /* loop until angle_degree > 360 */ { angle_radian = pi * angle_degree/180.0 ; value = sin(angle_radian); printf ( " %3d %f \n ", angle_degree, value ); angle_degree = angle_degree + 10; /* increment the loop index */ } }
well as its author. It is considered good programming style to identify and
document your work (although, sadly, most people only do this as an
afterthought). Comments can be written anywhere in the code: any characters
between /* and */ are ignored by the compiler and can be used to make the code
easier to understand. The use of variable names that are meaningful within the
context of the problem is also a good idea.
The #include statements now also include the header file for the
standard mathematics library math.h. This statement is needed to
define the calls to the trigonometric functions atan and
sin. Note also that the compilation
must include the mathematics library explicitly by typing
gcc sine.c -lm
Variable names are arbitrary (with some compiler-defined maximum length,
typically 32 characters). C uses the following standard variable types:
int -> integer variable short -> short integer long -> long integer float -> single precision real (floating point) variable double -> double precision real (floating point) variable char -> character variable (single byte)The compilers checks for consistency in the types of all variables used in
any code. This feature is intended to prevent mistakes, in particular in
mistyping variable names. Calculations done in the math library routines are
usually done in double precision arithmetic (64 bits on most workstations). The
actual number of bytes used in the internal storage of these data types depends
on the machine being used.
The printf function can be instructed to print integers, floats
and strings properly. The general syntax is
printf( "format", variables );where "format" specifies the converstion specification and
variables is a list of quantities to print. Some useful formats are
%.nd integer (optional n = number of columns; if 0, pad with zeroes)
%m.nf float or double (optional m = number of columns,
n = number of decimal places)
%ns string (optional n = number of columns)
%c character
\n \t to introduce new line or tab
\g ring the bell (``beep'') on the terminal
3. Loops
Most real programs contain some constructthat loops within the program, performing repetitive actions on a stream of data
or a region of memory. There are several ways to loop in C. Two of the most
common are the while loop:
while (expression)
{
...block of statements to execute...
}
and the for loop: for (expression_1; expression_2; expression_3)
{
...block of statements to execute...
}
The while loop continues to loop until the conditionalexpression becomes false. The condition is tested upon entering the
loop. Any logical construction (see below for a list) can be used in this
context.
The for loop is a special case, and is equivalent to the following
while loop:
expression_1;
while (expression_2)
{
...block of statements...
expression_3;
}
For instance, the following structure is often encountered: i = initial_i;
while (i <= i_max)
{
...block of statements...
i = i + i_increment;
}
This structure may be rewritten in the easier syntax of the forloop as:
for (i = initial_i; i <= i_max; i = i + i_increment)
{
...block of statements...
}
Infinite loops are possible (e.g. for(;;)), but not too goodfor your computer budget! C permits you to write an infinite loop, and provides
the break statement to ``breakout '' of the loop. For example,
consider the following (admittedly not-so-clean) re-write of the previous loop:
angle_degree = 0;
for ( ; ; )
{
...block of statements...
angle_degree = angle_degree + 10;
if (angle_degree == 360) break;
}
The conditional if simply asks whether angle_degreeis equal to 360 or not; if yes, the loop is stopped.
4. Symbolic Constants
You can define constants ofany type by using the #define compiler directive. Its syntax is
simple--for instance
#define ANGLE_MIN 0 #define ANGLE_MAX 360would define ANGLE_MIN and ANGLE_MAX to the values 0
and 360, respectively. C distinguishes between lowercase and uppercase letters
in variable names. It is customary to use capital letters in defining global
constants.
5. Conditionals
Conditionals are used withinthe if and while constructs:
if (conditional_1)
{
...block of statements executed if conditional_1 is true...
}
else if (conditional_2)
{
...block of statements executed if conditional_2 is true...
}
else
{
...block of statements executed otherwise...
}
and any variant that derives from it, either by omitting branches or byincluding nested conditionals.
Conditionals are logical operations involving comparison of quantities (of
the same type) using the conditional operators:
< smaller than <= smaller than or equal to == equal to != not equal to >= greater than or equal to > greater thanand the boolean operators
&& and || or ! not
Another conditional use is in the switch construct:
switch (expression)
{
case const_expression_1:
{
...block of statements...
break;
}
case const_expression_2:
{
...block of statements...
break;
}
default:
{
...block of statements..
}
}
The appropriate block of statements is executed according to the value ofthe expression, compared with the constant expressions in the case statement.
The break statements insure that the statements in the cases
following the chosen one will not be executed. If you would want to execute
these statements, then you would leave out the break statements. This
construct is particularly useful in handling input variables.
6. Pointers
The C language allows the programmerto ``peek and poke'' directly into memory locations. This gives great
flexibility and power to the language, but it also one of the great hurdles that
the beginner must overcome in using the language.
All variables in a program reside in memory; the statements
float x; x = 6.5;request that the compiler reserve 4 bytes of memory (on a 32-bit computer)
for the floating-point variable x, then put the ``value'' 6.5 in it.
Sometimes we want to know where a variable resides in memory. The address
(location in memory) of any variable is obtained by placing the operator
``&'' before its name. Therefore &x is the address
of x. C allows us to go one stage further and define a variable,
called a pointer, that contains the address of (i.e. ``points to'')
other variables. For example:
float x; float* px; x = 6.5; px = &x;defines px to be a pointer to objects of type float, and sets
it equal to the address of x:
The content of the memory location referenced by a pointer is obtained using
the ``*'' operator (this is called dereferencing the
pointer). Thus, *px refers to the value of x.
C allows us to perform arithmetic operations using pointers, but beware that
the ``unit'' in pointer arithmetic is the size (in bytes) of the object to which
the pointer points. For example, if px is a pointer to a variable
x of type float, then the expression px + 1
refers not to the next bit or byte in memory but to the location of the next
float after x (4 bytes away on most workstations); if
x were of type double, then px + 1 would refer
to a location 8 bytes (the size of a double)away, and so on. Only if
x is of type char will px + 1 actually refer to
the next byte in memory.
Thus, in
char* pc; float* px; float x; x = 6.5; px = &x; pc = (char*) px;(the (char*) in the last line is a ``cast'', which converts one
data type to another), px and pc both point to the same
location in memory--the address of x--but px + 1 and
pc + 1 point to different memory locations.
Consider the following simple code.
Run this code to see the results of these different operations. Note that,
void main() { float x, y; /* x and y are of float type */ float *fp, *fp2; /* fp and fp2 are pointers to float */ x = 6.5; /* x now contains the value 6.5 */ /* print contents and address of x */ printf("Value of x is %f, address of x %ld\n", x, &x); fp = &x; /* fp now points to location of x */ /* print the contents of fp */ printf("Value in memory location fp is %f\n", *fp); /* change content of memory location */ *fp = 9.2; printf("New value of x is %f = %f \n", *fp, x); /* perform arithmetic */ *fp = *fp + 1.5; printf("Final value of x is %f = %f \n", *fp, x); /* transfer values */ y = *fp; fp2 = fp; printf("Transfered value into y = %f and fp2 = %f \n", y, *fp2); }
while the value of a pointer (if you print it out with printf) is
typically a large integer, denoting some particular memory location in the
computer, pointers are not integers--they are a completely different
data type.
7. Arrays
Arrays of any type can be formed in C. Thesyntax is simple:
type name[dim];In C, arrays starts at position 0. The elements of the array occupy
adjacent locations in memory. C treats the name of the array as if it were a
pointer to the first element--this is important in understanding how to do
arithmetic with arrays. Thus, if v is an array, *v is the
same thing as v[0], *(v+1) is the same thing as
v[1], and so on:
Consider the following code, which illustrates the use of pointers:
(The expression ``i++'' is C shorthand for ``i = i +
#define SIZE 3 void main() { float x[SIZE]; float *fp; int i; /* initialize the array x */ /* use a "cast" to force i */ /* into the equivalent float */ for (i = 0; i < SIZE; i++) x[i] = 0.5*(float)i; /* print x */ for (i = 0; i < SIZE; i++) printf(" %d %f \n", i, x[i]); /* make fp point to array x */ fp = x; /* print via pointer arithmetic */ /* members of x are adjacent to */ /* each other in memory */ /* *(fp+i) refers to content of */ /* memory location (fp+i) or x[i] */ for (i = 0; i < SIZE; i++) printf(" %d %f \n", i, *(fp+i)); }
1''.) Since x[i] means the i-th element of the array
x, and fp = x points to the start of the x
array, then *(fp+i) is the content of the memory address i
locations beyond fp, that is, x[i].
8. Character Arrays
A string constant ,such as
"I am a string"is an array of characters. It is represented internally in C by the ASCII
characters in the string, i.e., ``I'', blank, ``a'',
``m'',... for the above string, and terminated by the special null
character ``\0'' so programs can find the end of the string.
String constants are often used in making the output of code intelligible
using printf ;
printf("Hello, world\n");
printf("The value of a is: %f\n", a);
String constants can be associated with variables. C provides the
char type variable, which can contain one character--1 byte--at a
time. A character string is stored in an array of character type, one ASCII
character per location. Never forget that, since strings are conventionally
terminated by the null character ``\0'', we require one extra storage
location in the array!
C does not provide any operator which manipulate entire strings at once.
Strings are manipulated either via pointers or via special routines available
from the standard string library string.h. Using character
pointers is relatively easy since the name of an array is a just a pointer to
its first element. Consider the following code:
The standard ``string'' library contains many useful functions to
void main() { char text_1[100], text_2[100], text_3[100]; char *ta, *tb; int i; /* set message to be an arrray */ /* of characters; initialize it */ /* to the constant string "..." */ /* let the compiler decide on */ /* its size by using [] */ char message[] = "Hello, I am a string; what are you?"; printf("Original message: %s\n", message); /* copy the message to text_1 */ /* the hard way */ i=0; while ( (text_1[i] = message[i]) != '\0' ) i++; printf("Text_1: %s\n", text_1); /* use explicit pointer arithmetic */ ta=message; tb=text_2; while ( ( *tb++ = *ta++ ) != '\0' ) ; printf("Text_2: %s\n", text_2); }
manipulate strings; a description of this library can be found in an appendix of
the K & R textbook. Some of the most useful functions are:
char *strcpy(s,ct) -> copy ct into s, including ``\0''; return s
char *strncpy(s,ct,n) -> copy ncharcater of ct into s, return s
char *strncat(s,ct) -> concatenate ct to end of s; return s
char *strncat(s,ct,n) -> concatenate n character of ct to end
of s, terminate with ``\0''; return s
int strcmp(cs,ct) -> compare cs and ct; return 0 if cs=ct,
<0 if cs0 if cs>ct
char *strchr(cs,c) -> return pointer to first occurence of c
in cs or NULL if not encountered
size_t strlen(cs) -> return length of cs
(s and t are char*, cs andct are const char*, c is an char
converted to type int, and n is an int.)
Consider the following code which uses some of these functions:
#include < string.h> void main() { char line[100], *sub_text; /* initialize string */ strcpy(line,"hello, I am a string;"); printf("Line: %s\n", line); /* add to end of string */ strcat(line," what are you?"); printf("Line: %s\n", line); /* find length of string */ /* strlen brings back */ /* length as type size_t */ printf("Length of line: %d\n", (int)strlen(line)); /* find occurence of substrings */ if ( (sub_text = strchr ( line, 'W' ) )!= NULL ) printf("String starting with \"W\" ->%s\n", sub_text); if ( ( sub_text = strchr ( line, 'w' ) )!= NULL ) printf("String starting with \"w\" ->%s\n", sub_text); if ( ( sub_text = strchr ( sub_text, 'u' ) )!= NULL ) printf("String starting with \"w\" ->%s\n", sub_text); }
9. I/O Capabilities
Character level I/O
C provides (through its libraries) a variety of I/Oroutines. At the character level, getchar() reads one character at a
time from stdin, while putchar() writes one character at a
time to stdout. For example, consider
This program counts the number of characters in the input stream (e.g. in
#include < stdio.h> void main() { int i, nc; nc = 0; i = getchar(); while (i != EOF) { nc = nc + 1; i = getchar(); } printf("Number of characters in file = %d\n", nc); }
a file piped into it at execution time). The code reads characters (whatever
they may be) from stdin (the keyboard), uses stdout (the
X-terminal you run from) for output, and writes error messages to
stderr (usually also your X-terminal). These streams are always
defined at run time. EOF is a special return value, defined in
stdio.h, returned by getchar() when it encounters an
end-of-file marker when reading. Its value is computer dependent, but
the C compiler hides this fact from the user by defining the variable EOF. Thus
the program reads characters from stdin and keeps adding to the
counter nc, until it encounters the ``end of file''.
An experienced C programmer would probably code this example as:
C allows great brevity of expression, usually at the expense of
#include < stdio.h> void main() { int c, nc = 0; while ( (c = getchar()) != EOF ) nc++; printf("Number of characters in file = %d\n", nc); }
readability!
The () in the statement (c = getchar()) says to execute
the call to getchar() and assign the result to c before
comparing it to EOF; the brackets are necessary here. Recall that
nc++ (and, in fact, also ++nc) is another way of writing
nc = nc + 1. (The difference between the prefix and postfix notation
is that in ++nc, nc is incremented before it is used,
while in nc++, nc is used before it is incremented. In
this particular example, either would do.) This notation is more compact (not
always an advantage, mind you), and it is often more efficiently coded by the
compiler.
The UNIX command wc counts the characters, words and lines in a
file. The program above can be considered as your own wc. Let's add
a counter for the lines.
Can you think of a way to count the number of words in the file?
#include < stdio.h> void main() { int c, nc = 0, nl = 0; while ( (c = getchar()) != EOF ) { nc++; if (c == '\n') nl++; } printf("Number of characters = %d, number of lines = %d\n", nc, nl); }
Higher-Level I/O capabilities
We have already seen thatprintf handles formatted output to stdout. The counterpart
statement for reading from stdin is scanf. The syntax
scanf("format string", variables);
resembles that of printf. The format string may contain blanksor tabs (ignored), ordinary ASCII characters, which must match those in
stdin, and conversion specifications as in printf.
Equivalent statements exist to read from or write to character strings. They
are:
sprintf(string, "format string", variables); scanf(string, "format string", variables);The ``string'' argument is the name of (i.e. a pointer to) the character
array into which you want to write the information.
I/O to and from files
Similar statements also exist for handling I/O toand from files. The statements are
#include < stdio.h>
FILE *fp;
fp = fopen(name, mode);
fscanf(fp, "format string", variable list);
fprintf(fp, "format string", variable list);
fclose(fp );
The logic here is that the code must- define a local ``pointer'' of type FILE (note that the
uppercase is necessary here), which is defined in < stdio.h>
- ``open'' the file and associate it with the local pointer via
fopen
- perform the I/O operations using fscanf and fprintf
- disconnect the file from the task with fclose
``mode'' argument in the fopen specifies the purpose/positioning in
opening the file: ``r'' for reading, ``w'' for writing,
and ``a'' for appending to the file. Try the following:
Compile and run this code; then use any editor to read the file
#include < stdio.h> void main() { FILE *fp; int i; fp = fopen("foo.dat", "w"); /* open foo.dat for writing */ fprintf(fp, "\nSample Code\n\n"); /* write some info */ for (i = 1; i <= 10 ; i++) fprintf(fp, "i = %d\n", i); fclose(fp); /* close the file */ }
foo.dat.
10. Functions
Functions are easy to use; theyallow complicated programs to be parcelled up into small blocks, each of which
is easier to write, read, and maintain. We have already encountered the function
main and made use of I/O and mathematical routines from the standard
libraries. Now let's look at some other library functions, and how to write and
use our own.
Calling a Function
The call to a function in C simply entailsreferencing its name with the appropriate arguments. The C compiler checks for
compatibility between the arguments in the calling sequence and the definition
of the function.
Library functions are generally not available to us in source form. Argument
type checking is accomplished through the use of header files (like
stdio.h) which contain all the necessary information. For example, as
we saw earlier, in order to use the standard mathematical library you must
include math.h via the statement
#include < math.h>at the top of the file containing your code. The most commonly used header
files are
< stdio.h> -> defining I/O routines
< ctype.h> -> defining character manipulation routines
< string.h> -> defining string manipulation routines
< math.h> -> defining mathematical routines
< stdlib.h> -> defining number conversion, storage allocation
and similar tasks
< stdarg.h> -> defining libraries to handle routines with variable
numbers of arguments
< time.h> -> defining time-manipulation routines
In addition, the following header files exist: < assert.h> -> defining diagnostic routines < setjmp.h> -> defining non-local function calls < signal.h> -> defining signal handlers < limits.h> -> defining constants of the int type < float.h> -> defining constants of the float typeAppendix B in the K & R book describes these libraries in great
detail.
Writing Your Own Functions
A function has thefollowing layout:
return-type function-name ( argument-list-if-necessary ) { ...local-declarations... ...statements... return return-value; }If return-type is omitted, C defaults to int. The
return-value must be of the declared type.
void function-name ( argument-list-if-necessary ) { ...local-declarations... ...statements... }
As an example of function calls, consider the following code:
/* include headers of library */ /* defined for all routines */ /* in the file */ #include < stdio.h> #include < string.h> /* prototyping of functions */ /* to allow type checks by */ /* the compiler */ void main() { int n; char string[50]; /* strcpy(a,b) copies string b into a */ /* defined via the stdio.h header */ strcpy(string, "Hello World"); /* call own function */ n = n_char(string); printf("Length of string = %d\n", n); } /* definition of local function n_char */ int n_char(char string[]) { /* local variable in this function */ int n; /* strlen(a) returns the length of */ /* string a */ /* defined via the string.h header */ n = strlen(string); if (n > 50) printf("String is longer than 50 characters\n"); /* return the value of integer n */ return n; }
Run this code and observe that a and b are NOT
#include < stdio.h> void exchange(int a, int b); void main() { /* WRONG CODE */ int a, b; a = 5; b = 7; printf("From main: a = %d, b = %d\n", a, b); exchange(a, b); printf("Back in main: "); printf("a = %d, b = %d\n", a, b); } void exchange(int a, int b) { int temp; temp = a; a = b; b = temp; printf(" From function exchange: "); printf("a = %d, b = %d\n", a, b); }
exchanged! Only the copies of the arguments are exchanged. The RIGHT way to do
this is of course to use pointers:
The rule of thumb here is that
#include < stdio.h> void exchange ( int *a, int *b ); void main() { /* RIGHT CODE */ int a, b; a = 5; b = 7; printf("From main: a = %d, b = %d\n", a, b); exchange(&a, &b); printf("Back in main: "); printf("a = %d, b = %d\n", a, b); } void exchange ( int *a, int *b ) { int temp; temp = *a; *a = *b; *b = temp; printf(" From function exchange: "); printf("a = %d, b = %d\n", *a, *b); }
- You use regular variables if the function does not change the values of
those arguments
- You MUST use pointers if the function changes the values of those
arguments
11. Command-line arguments
It is standardpractice in UNIX for information to be passed from the command line directly
into a program through the use of one or more command-line arguments, or
switches. Switches are typically used to modify the behavior of a
program, or to set the values of some internal parameters. You have already
encountered several of these--for example, the "ls" command lists the
files in your current directory, but when the switch -l is added,
"ls -l" produces a so-called ``long'' listing instead. Similarly,
"ls -l -a" produces a long listing, including ``hidden'' files, the
command "tail -20" prints out the last 20 lines of a file (instead of
the default 10), and so on.
Conceptually, switches behave very much like arguments to functions within C,
and they are passed to a C program from the operating system in precisely the
same way as arguments are passed between functions. Up to now, the
main() statements in our programs have had nothing between the
parentheses. However, UNIX actually makes available to the program (whether the
programmer chooses to use the information or not) two arguments to
main: an array of character strings, conventionally called
argv, and an integer, usually called argc, which
specifies the number of strings in that array. The full statement of the first
line of the program is
main(int argc, char** argv)(The syntax char** argv declares argv to be a pointer to a
pointer to a character, that is, a pointer to a character array (a character
string)--in other words, an array of character strings. You could also write
this as char* argv[]. Don't worry too much about the details of the
syntax, however--the use of the array will be made clearer below.)
When you run a program, the array argv contains, in order,
all the information on the command line when you entered the command
(strings are delineated by whitespace), including the command itself.
The integer argc gives the total number of strings, and is therefore
equal to equal to the number of arguments plus one. For example, if you
typed
a.out -i 2 -g -x 3 4the program would receive
argc = 7 argv[0] = "a.out" argv[1] = "-i" argv[2] = "2" argv[3] = "-g" argv[4] = "-x" argv[5] = "3" argv[6] = "4"Note that the arguments, even the numeric ones, are all strings
at this point. It is the programmer's job to decode them and decide what to do
with them.
The following program simply prints out its own name and arguments:
#include < stdio.h>
main(int argc, char** argv)
{
int i;
printf("argc = %d\n", argc);
for (i = 0; i < argc; i++)
printf("argv[%d] = \"%s\"\n", i, argv[i]);
}
UNIX programmers have certain conventions about how to interpret theargument list. They are by no means mandatory, but it will make your program
easier for others to use and understand if you stick to them. First, switches
and key terms are always preceded by a ``-'' character. This makes them easy to
recognize as you loop through the argument list. Then, depending on the switch,
the next arguments may contain information to be interpreted as integers,
floats, or just kept as character strings. With these conventions, the most
common way to ``parse'' the argument list is with a for loop and a
switch statement, as follows:
#include < stdio.h>
#include < stdlib.h>
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a_value = 0;
float b_value = 0.0;
char* c_value = NULL;
int d1_value = 0, d2_value = 0;
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i < argc; i++) {
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': a_value = atoi(argv[++i]);
break;
case 'b': b_value = atof(argv[++i]);
break;
case 'c': c_value = argv[++i];
break;
case 'd': d1_value = atoi(argv[++i]);
d2_value = atoi(argv[++i]);
break;
}
}
}
printf("a = %d\n", a_value);
printf("b = %f\n", b_value);
if (c_value != NULL) printf("c = \"%s\"\n", c_value);
printf("d1 = %d, d2 = %d\n", d1_value, d2_value);
}
Note that argv[i][j] means the j-th character of thei-th character string. The if statement checks for a
leading ``-'' (character 0), then the switch statement allows various
courses of action to be taken depending on the next character in the string
(character 1 here). Note the use of argv[++i] to increase
i before use, allowing us to access the next string in a single
compact statement. The functions atoi and atof are defined
in stdlib.h. They convert from character strings to ints
and doubles, respectively.
A typical command line might be:
a.out -a 3 -b 5.6 -c "I am a string" -d 222 111(The use of double quotes with -c here makes sure that the
shell treats the entire string, including the spaces, as a single object.)
Arbitrarily complex command lines can be handled in this way. Finally, here's
a simple program showing how to place parsing statements in a separate function
whose purpose is to interpret the command line and set the values of its
arguments:
/********************************/
/* */
/* Getting arguments from */
/* */
/* the Command Line */
/* */
/********************************/
/* Steve McMillan */
/* Written: Winter 1995 */
#include < stdio.h>
#include < stdlib.h>
void get_args(int argc, char** argv, int* a_value, float* b_value)
{
int i;
/* Start at i = 1 to skip the command name. */
for (i = 1; i < argc; i++) {
/* Check for a switch (leading "-"). */
if (argv[i][0] == '-') {
/* Use the next character to decide what to do. */
switch (argv[i][1]) {
case 'a': *a_value = atoi(argv[++i]);
break;
case 'b': *b_value = atof(argv[++i]);
break;
default: fprintf(stderr,
"Unknown switch %s\n", argv[i]);
}
}
}
}
main(int argc, char** argv)
{
/* Set defaults for all parameters: */
int a = 0;
float b = 0.0;
get_args(argc, argv, &a, &b);
printf("a = %d\n", a);
printf("b = %f\n", b);
}
12. Graphical Interfaces: Dialog Boxes
Supposeyou don't want to deal with command line interpretation, but you still want your
program to be able to change the values of certain variables in an interactive
way. You could simply program in a series printf/scanf lines to quiz
the user about their preferences:
.
.
.
printf("Please enter the value of n: ");
scanf("%d", &n);
printf("Please enter the value of x: ");
scanf("%f", &x);
.
.
.
and so on, but this won't work well if your program is to be used as part of
a pipeline (see the UNIX
primer), for example using ther graphics program plot_data,
since the questions and answers will get mixed up with the data stream.
A convenient alternative is to use a simple graphical interface which
generates a dialog box, offering you the option of varying key
parameters in your program. Our graphics package provides a number of
easy-to-use tools for constructing and using such boxes. The simplest way to set
the integer variable n and the float variable x (i.e. to
perform the same effect as the above lines of code) using a dialog box is as
follows:
/* Simple program to illustrate use of a dialog box */
main()
{
/* Define default values: */
int n = 0;
float x = 0.0;
/* Define contents of dialog window */
create_int_dialog_entry("n", &n);
create_float_dialog_entry("x", &x);
/* Create window with name "Setup" and top-left corner at (0,0) */
set_up_dialog("Setup", 0, 0);
/* Display the window and read the results */
read_dialog_window();
/* Print out the new values */
printf("n = %d, x = %f\n", n, x);
}
Compile this program using the alias Cgfx (see the page on compilation)to link in all necessary libraries.
The two create lines define the entries in the box and the
variables to be associated with them, set_up_dialog names the box and
defines its location. Finally, read_dialog_window pops up a window
and allows you to change the values of the variables. When the program runs, you
will see a box that looks something like this:
Modify the numbers shown, click "OK" (or just hit carriage return), and the
changes are made. That's all there is to it! The great advantage of this
approach is that it operates independently of the flow of data through
stdin/stdout. In principle, you could even control the operation of
every stage in a pipeline of many chained commands, using a separate dialog box
for each.
